The present invention relates generally to a heat-pump type air-conditioning apparatus and control method therefore, and more particularly to defrosting control of an outdoor heat exchanger of such a heat-pump type air-conditioning apparatus.
Generally, as a system for defrosting frost attached to an outdoor heat exchanger of a heat-pump type air-conditioning apparatus is known the so-called reverse-cycle defrosting system in which valve switching is formed so that the outdoor heat exchanger is used as a condenser and the room heat exchanger is used as an evaporator. Problems with this system are that the defrosting time is long because of small refrigerant circulation and heating has to be stopped during the defrosting operation and further a long time is required to elevate the temperature of the room heat exchanger after termination of the defrosting operation.
Attempts to avoid the problems noted above have been made heretofore, such a technique being disclosed as the so-called hot-gas bypass defrosting system in "Japan Refrigeration Association Lecture Report" S59-11, Page 53, for example, in which a portion of the delivery gas from the compressor is supplied by a predetermined quantity to the room heat exchanger for keeping a slight heating capability and the remainder of the delivery gas therefrom is introduced into the outlet side of the outdoor heat exchanger for further performing defrosting. Although satisfactory for elimination of the above-mentioned problems, such a defrosting system has disadvantages, an important one of which relates to reduction of the oil level within the compressor and reduction of reliability of the compressor attendant thereupon. More specifically, as shown in FIG. 5, in response to start of the defrosting operation, the operating frequency of the compressor is arranged to be quickly increased from a frequency immediately before the start of the defrosting operation to the maximum operating frequency and a portion of high-temperature delivery gas is bypassed through a bypass line to the outlet side of the outdoor heat exchanger. Therefore, the pressure difference between both the sides of the outdoor heat exchanger is momentarily reduced as shown in FIG. 6 and, as a result, the refrigerant is formed under the low pressure condition and the oil of the compressor is discharged together with the refrigerant to the outside of the compressor to result in abrupt reduction of the oil level. In addition, when the operating frequency of the compressor is then increased at a stretch to the maximum operating frequency, the oil thereof is further discharged so as to cause further reduction of the oil level, resulting in lowering of reliability of the compressor. Thus, a further improvement of the hot-gas bypass defrosting system would be required from the viewpoint of improvement of reliability of the compressor.